Fish completion using Go completion

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

bash_completions.md

@@ -115,6 +115,8 @@ in this example again instead of the replication controllers.
 
 In some cases it is not possible to provide a list of possible completions in advance.  Instead, the list of completions must be determined at execution-time.  Cobra provides two ways of defining such dynamic completion of nouns. Note that both these methods can be used along-side each other as long as they are not both used for the same command.
 
+**Note**: *Custom Completions written in Go* will automatically work for other shell-completion scripts (e.g., Fish shell), while *Custom Completions written in Bash* will only work for Bash shell-completion.  It is therefore recommended to use *Custom Completions written in Go*.
+
 #### 1. Custom completions of nouns written in Go
 
 In a similar fashion as for static completions, you can use the `ValidArgsFunction` field to provide a Go function that Cobra will execute when it needs the list of completion choices for the nouns of a command.  Note that either `ValidArgs` or `ValidArgsFunction` can be used for a single cobra command, but not both.
@@ -301,6 +303,8 @@ So while there are many other files in the CWD it only shows me subdirs and thos
 
 As for nouns, Cobra provides two ways of defining dynamic completion of flags.  Note that both these methods can be used along-side each other as long as they are not both used for the same flag.
 
+**Note**: *Custom Completions written in Go* will automatically work for other shell-completion scripts (e.g., Fish shell), while *Custom Completions written in Bash* will only work for Bash shell-completion.  It is therefore recommended to use *Custom Completions written in Go*.
+
 ## 1. Custom completions of flags written in Go
 
 To provide a Go function that Cobra will execute when it needs the list of completion choices for a flag, you must register the function in the following manner:

fish_completions.go

@@ -0,0 +1,172 @@
+package cobra
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+)
+
+func genFishComp(buf *bytes.Buffer, name string, includeDesc bool) {
+	compCmd := CompRequestCmd
+	if includeDesc {
+		compCmd = CompWithDescRequestCmd
+	}
+	buf.WriteString(fmt.Sprintf("# fish completion for %-36s -*- shell-script -*-\n", name))
+	buf.WriteString(fmt.Sprintf(`
+function __%[1]s_debug
+    set file "$BASH_COMP_DEBUG_FILE"
+    if test -n "$file"
+        echo "$argv" >> $file
+    end
+end
+
+function __%[1]s_perform_completion
+    __%[1]s_debug "Starting __%[1]s_perform_completion with: $argv"
+
+    set args (string split -- " " "$argv")
+    set lastArg "$args[-1]"
+
+    __%[1]s_debug "args: $args"
+    __%[1]s_debug "last arg: $lastArg"
+
+    set emptyArg ""
+    if test -z "$lastArg"
+        __%[1]s_debug "Setting emptyArg"
+        set emptyArg \"\"
+    end
+    __%[1]s_debug "emptyArg: $emptyArg"
+
+    set requestComp "$args[1] %[2]s $args[2..-1] $emptyArg"
+    __%[1]s_debug "Calling $requestComp"
+
+    set results (eval $requestComp 2> /dev/null)
+    set comps $results[1..-2]
+    set directiveLine $results[-1]
+
+    # For Fish, when completing a flag with an = (e.g., <program> -n=<TAB>)
+    # completions must be prefixed with the flag
+    set flagPrefix (string match -r -- '-.*=' "$lastArg")
+
+    __%[1]s_debug "Comps: $comps"
+    __%[1]s_debug "DirectiveLine: $directiveLine"
+    __%[1]s_debug "flagPrefix: $flagPrefix"
+
+    for comp in $comps
+        printf "%%s%%s\n" "$flagPrefix" "$comp"
+    end
+
+    printf "%%s\n" "$directiveLine"
+end
+
+# This function does three things:
+# 1- Obtain the completions and store them in the global __%[1]s_comp_results
+# 2- Set the __%[1]s_comp_do_file_comp flag if file completion should be performed
+#    and unset it otherwise
+# 3- Return true if the completion results are not empty
+function __%[1]s_prepare_completions
+    # Start fresh
+    set --erase __%[1]s_comp_do_file_comp
+    set --erase __%[1]s_comp_results
+
+    # Check if the command-line is already provided.  This is useful for testing.
+    if not set --query __%[1]s_comp_commandLine
+        set __%[1]s_comp_commandLine (commandline)
+    end
+    __%[1]s_debug "commandLine is: $__%[1]s_comp_commandLine"
+
+    set results (__%[1]s_perform_completion "$__%[1]s_comp_commandLine")
+    set --erase __%[1]s_comp_commandLine
+    __%[1]s_debug "Completion results: $results"
+
+    if test -z "$results"
+        __%[1]s_debug "No completion, probably due to a failure"
+        # Might as well do file completion, in case it helps
+        set --global __%[1]s_comp_do_file_comp 1
+        return 0
+    end
+
+    set directive (string sub --start 2 $results[-1])
+    set --global __%[1]s_comp_results $results[1..-2]
+
+    __%[1]s_debug "Completions are: $__%[1]s_comp_results"
+    __%[1]s_debug "Directive is: $directive"
+
+    if test -z "$directive"
+        set directive 0
+    end
+
+    set compErr (math (math --scale 0 $directive / %[3]d) %% 2)
+    if test $compErr -eq 1
+        __%[1]s_debug "Received error directive: aborting."
+        # Might as well do file completion, in case it helps
+        set --global __%[1]s_comp_do_file_comp 1
+        return 0
+    end
+
+    set nospace (math (math --scale 0 $directive / %[4]d) %% 2)
+    set nofiles (math (math --scale 0 $directive / %[5]d) %% 2)
+
+    __%[1]s_debug "nospace: $nospace, nofiles: $nofiles"
+
+    # Important not to quote the variable for count to work
+    set numComps (count $__%[1]s_comp_results)
+    __%[1]s_debug "numComps: $numComps"
+
+    if test $numComps -eq 1; and test $nospace -ne 0
+        # To support the "nospace" directive we trick the shell
+        # by outputting an extra, longer completion.
+        __%[1]s_debug "Adding second completion to perform nospace directive"
+        set --append __%[1]s_comp_results $__%[1]s_comp_results[1].
+    end
+
+    if test $numComps -eq 0; and test $nofiles -eq 0
+        __%[1]s_debug "Requesting file completion"
+        set --global __%[1]s_comp_do_file_comp 1
+    end
+
+    # If we don't want file completion, we must return true even if there
+    # are no completions found.  This is because fish will perform the last
+    # completion command, even if its condition is false, if no other
+    # completion command was triggered
+    return (not set --query __%[1]s_comp_do_file_comp)
+end
+
+# Remove any pre-existing completions for the program since we will be handling all of them
+# TODO this cleanup is not sufficient.  Fish completions are only loaded once the user triggers
+# them, so the below deletion will not work as it is run too early.  What else can we do?
+complete -c %[1]s -e
+
+# The order in which the below two lines are defined is very important so that __%[1]s_prepare_completions
+# is called first.  It is __%[1]s_prepare_completions that sets up the __%[1]s_comp_do_file_comp variable.
+#
+# This completion will be run second as complete commands are added FILO.
+# It triggers file completion choices when __%[1]s_comp_do_file_comp is set.
+complete -c %[1]s -n 'set --query __%[1]s_comp_do_file_comp'
+
+# This completion will be run first as complete commands are added FILO.
+# The call to __%[1]s_prepare_completions will setup both __%[1]s_comp_results abd __%[1]s_comp_do_file_comp.
+# It provides the program's completion choices.
+complete -c %[1]s -n '__%[1]s_prepare_completions' -f -a '$__%[1]s_comp_results'
+
+`, name, compCmd, BashCompDirectiveError, BashCompDirectiveNoSpace, BashCompDirectiveNoFileComp))
+}
+
+// GenFishCompletion generates fish completion file and writes to the passed writer.
+func (c *Command) GenFishCompletion(w io.Writer, includeDesc bool) error {
+	buf := new(bytes.Buffer)
+	genFishComp(buf, c.Name(), includeDesc)
+	_, err := buf.WriteTo(w)
+	return err
+}
+
+// GenFishCompletionFile generates fish completion file.
+func (c *Command) GenFishCompletionFile(filename string, includeDesc bool) error {
+	outFile, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	defer outFile.Close()
+
+	return c.GenFishCompletion(outFile, includeDesc)
+}

fish_completions.md

@@ -0,0 +1,7 @@
+## Generating Fish Completions for your own cobra.Command
+
+Cobra supports native Fish completions generated from the root `cobra.Command`.  You can use the `command.GenFishCompletion()` or `command.GenFishCompletionFile()` functions. You must provide these functions with a parameter indicating if the completions should be annotated with a description; Cobra will provide the description automatically based on usage information.  You can choose to make this option configurable by your users.
+
+### Limitations
+
+* Custom completions implemented using the `ValidArgsFunction` and `RegisterFlagCompletionFunc()` are supported automatically but the ones implemented in Bash scripting are not.